Ferroelectric-ferroelastic properties of K3Fe5F15 and the phase transition at 490 K

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Abstract
K3Fe5F15 has previously been predicted to be both ferroelectric and ferroelastic, with a phase transition at 535 K, on the basis of the atomic coordinates given in Acta Crystallogr. Sect. B 2 9, 1654 (1973). Subsequently, the dielectric permittivity has been found to reach a maximum at 495(10) K as the dielectric loss undergoes a change in slope, characteristic of ferroelectric behavior. Furthermore, the heat capacity exhibits a λ‐type anomaly at 490(10) K, with a corresponding entropy change of ΔS=5.5(2) J mol−1 K−1. The entropy change at the phase transition calculated from the predicted change in structure is 5.42 J mol−1 K−1. Ferroelastic domains present at room temperature disappear sharply on heating above 490(10) K, as K3Fe5F15 transforms from orthorhombic to tetragonal symmetry, and reappear on cooling below 480(10) K. The ferroelectric‐ferroelastic properties in the orthorhombic phase are shown to be fully coupled.